I take it this is referring to the 'build-a-beach' event, specifically the bit saying:

'Give her sand every day, and she'll give you a large mystery box!'

I'm hoping Monday isn't required for the 'every day' condition, as the event ends at game update (which can happen at any time) and not midnight. This could catch out many players, especially those that cannot log on early on Monday, who may find themselves in the same position as you.

Personally, I'd make Monday morning the 'catch-up' day, so to speak, so provided players contribute sand on 4 out of the 4 ½ days, they get the large mystery box reward.